I know this is the Australian Forum. I haven't quiet wprked out this website yet lol I did some noodling and came across this 2000 5 cent which apears to be copper? Could this coin have been toned to make it look older, or is this maybe a 5 cent struck on a wrong planchet
These are cupro-nickel coins, so they will go brown if buried in the soil for a while. It is probably a metal detector find. I've dug up plenty of 1947-67 British sixpences, which have the same metal composition as this coin, with exactly the same colouring.